1. Field of the Invention
The present invention relates to the process for epoxidation of olefins with an organic hydroperoxide, in the liquid phase with an organic solvent, in the presence of a soluble epoxidation catalyst. More particularly, the present invention relates to an improved epoxidation process including: oxidation of an olefin with a hydroperoxide, typically propylene with tertiary butyl hydroperoxide, in a primary reaction zone in liquid phase with an organic solvent, typically tertiary butyl alcohol, under epoxidation conditions for conversion of about 85 to 95% of the tertiary butyl hydroperoxide and for production of a first epoxidation reaction product; fractionation of the primary epoxidation reaction product into a first distillate fraction comprising unreacted propylene and propylene oxide, and into a primary liquid fraction comprising molybdenum catalyst, unreacted tertiary butyl hydroperoxide, tertiary butyl alcohol, and side reaction products; reaction of the primary liquid fraction with propylene in a secondary epoxidation zone under epoxidation conditions for conversion of about 85 to about 95% of the tertiary butyl hydroperoxide charged in the first liquid fraction, and for production of a secondary epoxidation reaction product; fractionation of the secondary epoxidation reaction product into a second distillate fraction comprising unreacted propylene and propylene oxide and into a secondary liquid fraction comprising molybdenum catalyst, tertiary butyl alcohol, and side reaction products; and recovering propylene and propylene oxide product from the primary and secondary distillate fractions.
2. Description of Pertinent Art
The epoxidation reactions contemplated herein are epoxidation reactions of the type disclosed by Kollar, U.S. Pat. No. 3,351,653, as have been elaborated, for example, in Marquis, et al., U.S. Pat. No. 4,891,437, where a wide variety of olefins can be epoxidized using a wide variety of organic hydroperoxides in the presence of catalytic metallic compounds.
Processes for epoxidation of C.sub.3 -C.sub.20 olefin hydrocarbons with an organic hydroperoxide in the presence of an epoxidation catalyst selected from compounds of molybdenum titanium, tungsten, vanadium and selenium to produce olefin oxide and an alcohol corresponding to the hydroperoxide are well known. Typically, commercial reactions employ propylene as olefin and tertiary butyl hydroperoxide as organic hydroperoxide. Products of these reactions, propylene oxide and tertiary butyl alcohol, are valuable intermediate chemicals for the manufacture of other products such as synthetic polymers and tertiary butyl ether.
In British Patent Specification No. 1,298,253, filed Jul. 14, 1970, a method is disclosed for the continuous epoxidation of propylene with tertiary butyl hydroperoxide in the presence of a molybdenum catalyst, which process comprises: reacting propylene in excess with tertiary butyl hydroperoxide in the presence of tertiary butyl alcohol solvent and molybdenum catalyst in a first epoxidation zone; fractionating effluent from the first reaction zone to yield an overhead fraction containing unreacted propylene, propylene oxide and tertiary butyl alcohol, and yield a bottoms fraction containing unconverted tertiary butyl hydroperoxide, the remainder of the tertiary butyl alcohol and molybdenum catalyst; separating the bottoms fraction into a recycle stream and a purge stream and recycling the recycle stream to said first epoxidation zone; reacting the purge stream in a second epoxidation zone with propylene; and recovering the propylene oxide product from each of said epoxidation zones. The British Specification claims as advantages for this process that consumption of catalysts is extremely low, tertiary butyl hydroperoxide conversion is high, and at the same time selectivity (i.e., moles of propylene oxide produced per mole of tertiary butyl hydroperoxide converted) is high, thus providing a high yield of propylene oxide based upon the amount of tertiary butyl hydroperoxide charged to the process. A review of the examples shows selectivity of tertiary butyl hydroperoxide conversion to propylene oxide is 57% without the second stage reaction of the present invention and 66% with the second stage reaction.
In British Specification 1,298,253, a large recycle stream containing catalyst and side reaction products is returned to the first epoxidation zone where it is brought into contact with the epoxidation reaction mixture, including propylene oxide as well as propylene and tertiary butyl hydroperoxide reactants. This recycle stream, in addition to containing molybdenum catalyst and unconverted tertiary butyl hydroperoxide, also contains side reaction products including low molecular weight (C.sub.1 -C.sub.4) carboxylic acids. These carboxylic acids catalyze additional side reactions including propylene dimerization reactions and propylene oxide-tertiary butyl hydroperoxide esterification and etherification reaction the products of which reduce selectivity, and yield of desired product. The purge stream, which forms the charge stock to the second reactor in the disclosed process, must of necessity remove the side reaction products at a rate equivalent to the rate at which they are formed. Since the purge stream is, as disclosed in the specification, only 8 to 20% of the heavy liquid stream from the distillation zone, the recycle stream returns to the first reaction zone from 5 to 12 times the amount of side reaction products, including the low molecular weight acids, as are produced in the initial reaction.
Stein, et al, in U.S. Pat. No. 3,849,451, issued Nov. 19, 1974, discloses a process for a catalytic epoxidation of olefinically unsaturated compounds employing organic hydroperoxides as epoxidizing agents and employing catalysts comprised of metals such as vanadium, tungsten, molybdenum, titanium and selenium. In Stein, et al., an epoxidation reaction is carried out under autogenous pressure and at a reaction temperature sufficient to volatilize a portion of the liquid phase reaction medium, including a portion of the olefin oxide product. Preferably from 1/3 to 1/2 of the olefin oxide product and substantially none of the organic hydroperoxide is removed with the volatilized portion. Preferably, the reaction zone is partitioned into several compartments with a common overhead zone, or alternatively, several reactors in series are employed to prevent back mixing of epoxidation reaction product with reactants entering the process. In Stein, et al., the unreacted olefin and a portion (1/3 to 1/2) of the olefin oxide product are vaporized and withdrawn from each compartment or from each series reactor. Olefin recovered from the withdrawn vapor is returned as reactant to each compartment or each series reactor. Stein, et al. recognizes that the olefin oxide product enters into side reactions catalyzed by the acidic side reaction products of the epoxidation reaction. However, in the process of Stein, et al., only a portion (1/3 to 1/2) of the olefin oxide is removed from each reactor compartment or reactor in series, leaving a substantial portion (1/3 to 2/3) of the olefin oxide in contact with the acidic compounds under conditions favorable for side reactions which consume olefin oxide.
Sweed, in U.S. Pat. No. 4,455,283, issued Jun. 19, 1984, discloses a process for epoxidizing olefin compounds with organic hydroperoxides in the presence of liquid solutions of dissolved molybdenum catalysts, and for recovery and recycle of the molybdenum catalyst values. In the description of the prior art, (column 2, lines 16-22) Sweed notes that distillation residue containing spent molybdenum catalyst, some alcohol, and acids as well as high boiling organic residues can be recycled directly to the epoxidation reaction zone, but direct recycle of the residue results in a build up within the system of impurities (e.g. acids) which are deleterious to subsequent epoxidation reactions.
Isaacs, in U.S. Pat. No. 4,598,057, issued Jul. 1, 1986, discloses a process for epoxidation of olefins with organic hydroperoxides in the presence of a molybdenum catalyst with subsequent recovery of molybdenum from a spent catalyst stream derived from the epoxidation reaction product. In the detailed description of the invention at column 4, lines 57-68, Isaacs states, "As indicated above, this heavy fraction cannot be recycled directly to the epoxidation zone in view of the fact that the impurities contained therein, and most notably the acid impurities, interfere with the epoxidation reaction. The deleterious effect of these acids is particularly pronounced in a continuous system due to a build up of the concentration of these materials when a direct recycle is employed. Furthermore, partial recycle of the stream to the epoxidation reaction, over a period of time, results in accumulation of residual materials associated with the catalyst which likewise is deleterious to the overall epoxidation reaction."
From the above references, it is seen that reactions for epoxidation of olefin using organic hydroperoxide in the presence of selected metal catalysts, such as molybdenum, are well known. Also, it is recognized that reactions catalyzed by acid side reaction products consume olefin oxide product and organic hydroperoxide reactant, thus reducing yield from the epoxidation reactions. Efforts, as disclosed in U.S. Pat. No. 3,849,451 and British Specification 1,298,253 discussed above, employing more than one reactor in the epoxidation process have been made to reduce the consumption of propylene oxide and organic hydroperoxide in unwanted side reactions. These efforts have not been completely successful. Either through recycle of acidic side reaction products to the main reactor, or through incomplete removal of olefin oxide from charge to a second reactor, the olefin oxide continued to remain in contact with acidic side reaction products. These acidic side reaction products catalyze reactions such as etherification and esterification reactions, which destroy the olefin oxide product and organic hydroperoxide. Consequently, improvements to the epoxidation process which will improve hydroperoxide conversion and increase product yields without increasing side reactions which consume olefin oxide and organic hydroperoxide, are desirable.